A review of measured transmembrane potentials (Em) in a number of cell types (eggs, normal and tumor cells, embryonic and post-natal tissues) shows that a large Em (-60 to -90 mV) correlates with absence of growth and infrequent cell divisions; low Em (-4 to -40 mV) correlates with intense growth and frequent cell divisions. We postulate that cell membrane depolarization (low Em) is a trigger or modulator of the cell cycle in both normal and tumor cells. In order to test this hypothesis we propose a procedure (a "2-dimensional dye probe"), based on the measurement of the voltage-dependent light emission of fluorescent dyes that bind to the cell membrane. Light emission will be detected by a video system with an image intensifier and tranformed into a voltage map using a digital computer. This would permit the mapping of electrophysiological variables onto the cell morphology in a number of cell lines (normal and tumor) growing in vitro. This technique will make possible a correlation of Em with cell cycle, growth rate, contact inhibition and malignancy. Because low Em is associated with low electrical coupling (EC) we will also study EC as a function of cell cycle, growth rate, contact inhibition and malignancy. Using this new method, it will be possible to obtain time- lapse images of the microscopic and the electrophysiological "morphology" (isopotential maps) of groups of cells. The method will be fast, simple in use, and will have numerous biological applications, (for example the determination of electrical activity in complex isolated nerve cell networks). If Em proves to be a trigger or modulator of the cell cycle, it might ultimately be possible to inhibit tumor growth in humans by selectively controlling Em.